Based on everything I've ever heard about, read about, and experienced, the biggest deal regarding personal safety when it comes to bears in the USA is whether or not you're in grizzly territory. Black bears, especially in lower latitudes, are relatively small...a 300 lb. Black Bear in CO is considered big. Now is that 300 pounds of bear I'd like to wrestle with? No, but as others have stated 99% of black bear encounters are them running away from us. Grizzlies aren't as easily scared off, and if they attack you're basically dead already unless you happen to be a good shot AND get lucky with either bear spray or proper firearm (12 gauge, .44 Mag and up revolver) to bring down what is basically the equivilent of least 2 black bears with rabies as far as power and viciousness goes. I still carry in Colorado for black bears but I personally think my .40 SW is enough to scare off any black bear. I've seen black bears 4 times in 12 years and they've always been jogging away from me even when I'm not being loud hiking alone. Before I even had the common sense to cook away from my tent and keep food in a tree overnight I never had a black bear enter my camp that I knew of. I've never been in grizz territory though I want to go to Yellowstone next summer.
Basically I hardly worry at all about black bears when it comes to personaly safety...I carry first and foremost because of the two legged predator. Now they definitely are willing to be a nuisance. I had a friend who left a loaf of bread in the trunk of his Saturn in Golden, CO (Denver metro area) and one tore the door about damn near off and then eventually got through the back seats into the trunk. All in the dead of night about 25 yards from his house. Like any inquisitive animal, they try to get away with as much as they can when they don't see or smell humans. If my friend had been sleeping in that car, the bear probably wouldn't have even approached it, IMO, and if he did he could have easily run away if my friend made a ton of noise and movement. Most animals don't take a lot of convincing, except the ones that know they can own us real good. A normal sized black bear versus Joe Schmo with a bowie knife is practically a fair fight.